As a power supply device that generates an output voltage higher than or lower than an input voltage, a comparator type direct-current to direct-current (DC-DC) converter is known according to U.S. published patent application No. 2007/0120547. For example, a step-down DC-DC converter on/off controls a switch circuit to which an input voltage is supplied and a current that flows through a coil coupled to the switch circuit is smoothed by a smoothing capacitor to generate an output voltage. The output voltage generated in the above described manner includes a ripple voltage (ripple component) caused by the coil current and equivalent series resistance (ESR) of the smoothing capacitor. Thus, the DC-DC converter compares the output voltage and a given standard voltage and turns on the switch circuit when the output voltage becomes lower than the standard voltage due to the ripple component, and thereby controls the output voltage.
As described above, the DC-DC converter that generates an output voltage by switching the switching circuit demands stabilization of the output voltage, in other words, an output voltage with fewer ripple components. In order to meet the demand, DC-DC converters that use a smoothing capacitor with small ESR have been developed.
However, a feed back system and a control cycle of a switch circuit become unstable when a value of ESR of a smoothing capacitor is made small. As a result, there is a drawback in that a voltage VL is irregularly applied to a coil and a current IL that flows through the coil changes irregularly, and thereby an output voltage Vo fluctuates as illustrated in FIG. 8.